Servo system



Nov. 3, 1953 P. J. JACKSON sERvo SYSTEM Filed May,l0, 1950 Patented Nov. 3, 1953 rasant SERVO SYSTEM Patrick John Jackson, Wembley, England, as-

signor to Hazeltine Research, Inc., Chicago, Ill., a corporation of Illinois Application May 10, 1950, Serial N o. 161,117

Claims priority, application Great Britain May 26, 1949 1 Claim.

1 GENERAL The present invention relates to servo systems and particularly to servo systems of the type which utilize a synchronous motor for driving the tuning element of a tunable oscillator automatically to control the frequency of the output signal thereof. The system is particularly suited for use in distance-measuring equipment and accordingly will be described in that environment.

Heretofore, servo systems adapted for use for automatic frequency control of a tunable oscillator have generally utilized a single motor as the driving means for the tuning element of the oscillator. In such systems, the motor ordinarily rotates only when it is necessary to retune the oscillator. Consequently, the initial inertia of the system generally may cause an undesirable time delay in effecting the retuning. Also, when tuning the oscillator, the motor tends to overrun. Reversal of rotation of the motor may therefore be necessary, thus causing the system to hunt for the desired frequency. Furthermore, due to the frequent interrupted rotation of the motor in prior such systems, undesirable jamming of the moving elements sometimes occurs. Accordingly, such systems have not been entirely satisfactory for some applications.

It i-s an object of the present invention, therefore, to provide a new and improved servo system which avoids one or more of the above-mentioned disadvantages of prior such systems.

It is another object of the invention to provide a new and improved servo system which is effective with only a slight time delay to adjust the output -signal of a tunable oscillator to a desired frequency.

It is a further object of the invention to provide a relatively simple servo system which has a reduced tendency to hunt and which is adapted for use in distance-,measuring equipment automatically to control the frequency of the output signal thereof.

In accordance with a particular form of the invention, a servo system for distance measuring equipment comprises a crystal-controlled oscillator for supplying a rst signal having a substantially constant frequency and a tunable oscillator for supplying a second signal including a frequency-adjusting device. The servo system includes a pair of continuously rotatable syn-v chronous motors and a differential gear mechanism for differentially connecting the pair of synchronous motors to the aforesaid adjusting (Cl. Z50-36) device to control the frequency of the second signal. The servo system also includes a pair of variable-frequency oscillators including a pair of frequency-determiningnetworks and coupled to the energizing circuits of the pair of synchronous motors for providing a pair of output signals to energize the pair of synchronous motors. The servo system additionally includes a frequency converter coupled to the crystal-controlled oscillator and the tunable oscillator for heterodyning the first and second signals to provide a signal having a frequency equal to the frequency difference between the rst and second signals and a frequency detector -coupled to the frequency converter and the pair of variablefrequency oscillators for deriving a control effect from the difference-frequency signal. The servo system also includes a pair of variable-resistance arnplilers coupled between the frequency detector and the pair of frequency-determining networks for utilizing the aforesaid control effect to cause the pair of variable-frequency oscillators so to control the frequencies of the output signals thereof as to determine the rotational speeds of the synchronous motors and thereby provide a predetermined frequency difference between the first and second signals including a frequency-adjusting device. The system further includes means responsive to the aforesaid signal for deriving a control effect varying with frequency variations of the above-mentioned signal from a predetermined frequency. The system also includes a pair of driving devices and circuit means for energizing the pair of driving devices. The above-mentioned circuit means includes at least one signal source responsive to the aforesaid control effect for energizing one of the driving devices. The servo system further includes a diflerential mechanism connecting the driving devices to the frequency-adjusting device to control the frequency of the aforesaid signal.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claim.

DESCRIPTION OF SERVO SYSTEM Referring now to the drawing, the figure is a circuit diagram, partly schematic, of a complete transmitter system for a distance-measuring equipment, which transmitter system embodies the present invention in a particular form. The servo system thereof comprises a means for supassai/ie e: plying a first signal, namely a local oscillator I which is preferably crystal-controlled. Ordinarily one of several crystals included in the os` cillator I0 may be selected to determine the output signal frequency thereof; The local oscillator I 0 also may include one or mre suitable stages of frequency multiplication to provide in the output circuit thereof a first signal preferably h'aving a substantially constant frequencyi l The servo system further comprises means including a tunable master oscillator Il for supplying in the output circuit thereof a second signal. The master oscillator lI includes means, represented diagrammatically as a rotatably adjustable condenser I2, for adjusting the frequency of the second signal. Although the adjusting means is represented for simplicity as the condenser I2, it may comprise any one of a number of conventional tuning elements adapted to tune the oscillator II throughout the operating frequency range thereof. Ordinarily the frequency of the second signal of the master oscillator I I` is different from the frequency of the nrst signal from the local oscillator ID. The tunable master oscillatcr. i I also may include conventional means for pulse-modulating the output signal thereof. The output circuit of the oscillator II is coupled toa conventional antenna system it, I3 and also is coupled by a conductor 40 to the input circuit of a frequency converter It.

The frequency converter It is preferably a crystal mixer in which an input circuit thereof is also coupled to the local oscillator ifi. The output circuit of the frequency converter Iii is coupled to a conventional intermediate-frequency amplilier I5 for supplying thereto an intermediate-frequency signal having a frequency equal to the frequency difference between the above-mentioned first and second signals.

The intermediate-frequency amplifier I5 is coupled to a frequency detector i6 which is preferably of the well-known balanced frequency discriminator type. its output terminals balanced with reference to ground for deriving` a control effect from the intermediateor difference-frequency signal which is applied thereto by the intermediate-frequency amplifier l5.

The output circuit of the frequency detector IB is coupled to` a conventional variable-frequency oscillator I8' which comprises an oscillator tube I9 and a variable-resistance amplifier including atube Il. The oscillator I8 is 0f the resistor-condenser type and to this end also includes a frequency-determining network comprising resistors 2| and 22 and condensers 23 and 24 for causing the output signal of the oscillator I8 to have a frequency which is ordinarily low with relation to the frequencies of the above-mentioned first and second signals. The oscillator I8 is energized from a suitable space-current source +B which is preferably of the regulated type. rThe4 input and output circuits of the amplifier' ll are conventional and the output circuit thereof is coupled to the frequency-determining network 2i, 22, 23, M for utilizing the derived control effect in the variable-frequency oscillator I8 to control the frequency of the output signal thereof. The output circuit of the oscillator I8` is coupled to a conventional low-frequency amplifier 25 of one or more stages which is in turn coupled to the energizing circuit of a driving means comprising preferably a continuously rotatable synchronous motor 26.

An output circuit of the frequency detector I6 The frequency detector l5 hasis also coupled to a variable-frequency oscillator 28 through a variable-resistance amplifier 2l. lThe oscillator 28 and the amplifier 2l are preferably similar to the oscillator i8 and the amplifier I'I, respectively. The output. circuit of the oscillator 28 is coupled through a. low-frequency amplifier 2Q to a synchronous motor 3i), both of the latter being similar to the corresponding units 25 and 2B, respectively, in the other branch. Hence, the means for energizing the pair of synchronous motors 26 and 30 comprise the pair of variable-frequency oscillators I8 and 28 which provide a` pair of output signals for energizing those motors.

From. the foregoing description it may be seen that a means is coupled to the local oscillator In, the tunable master oscillator I I, and the variable-frequency oscillators I8 and 28 for derivingacontrol effect related to a frequency variation of one of the above-mentioned first and second signals with reference` tothe other thereof. This means includes the frequency converter I4, the intermediate-frequency amplifier I5, andthe frequency detector I26.

The shafts of the motors 26 and 3U are mechanically connected toa differential gear mechanism 32 as represented by the dot-dash lines 3fI and 34, respectively. The differential gear mechanism 32 also is mechanically connected to the rotor of condenser I2 as represented by they dot-'- dash line 33. The mechanism 32 thereby differ`- entially connects thev motors 26- and' 30 to the condenser I`2 to controll the frequency of' the seeond signal mentioned above.

OPERATION OF SERVO SYSTEM.

Considering now the operation of the servo system just described, it. will be clear that thelocal oscillator I determines the frequency of the first signal applied to the frequency converter I4. Assume for the moment that the master oscillator I:I supplies to the frequency converter it a sec-- ond signal having a desired frequency higher than the frequency of the. first signal applied thereto. The firstv andY second signals-applied to the converter' are heterodyned therein to provide in the. output circuit of theI unit. Ill a desired `difference-frequency output signal within the' operating frequency range of the intermediate-frequency amplifier t5. rlhis output signal is'- applied to the amplifier I5 where it is amplified and applied to the, frequency detector I6. Preferably', the frequency of this difference-frequency signal is substantially the center frequencyl of the frequency detector I6. detector IB develops substantially the same potential at each ofv the output terminals thereof' and at each of the control electrodes of the variable-resistance amplifiers I1: and 21.

With the control electrodes of the variableresistance amplifiers IT and 21v at substantially the same potential the variable-frequency oscillators I8 and 251 may be initially adjusted to resonate at the same frequency by suitable selection of circuit components. They output signalsrof the oscillators I8 and 28 are applied to the low-frequency amplifiers 25` and 29-p respectively, are amplified therein and then supplied to the energizing circuits of the synchronous motors 26 and 30, respectively. Accordingly, these output signals cause the synchronous motors 26 and 3D continuously to rotate at the same speed. Hence, while the motors continuously rotate at the same4 speed; the differential connection of each thereof tothe rotor of the con.-

Accordingly, the frequencyY denser I 2 provided by the differential gear mechanism 32 causes the rotor of the condenser I2 to remain stationary. Thus, during this mode of operation the tuning of the condenser I2 is unchanged and an output signal having a desired frequency is applied to the antenna I3 by the master oscillator II.

Assuming now that it is desired to change the operating frequency of the master oscillator II to a lower frequency, this change may be accomplished through the operation of the servo system by selecting a suitable crystal for the local oscillator I0. The selection of a crystal having a frequency lower than the above-mentioned crystal causes a first signal having a frequency lower than the frequency of the rst signal mentioned above to be applied to the frequency converter I4 by the local oscillator II). Accordingly, the first and second signals applied to the frequency converter I4 by units I and I I beat together and provide a differencefrequency signal having a frequency higher than the frequency of the difference-frequency signal mentioned above. This difference-frequency signal is amplified by the intermediate-frequency amplifier I5 and thence applied to the frequency detector I6 which derives therefrom in its output circuit a control effect related to the frequency variation of the first signal with reference to the second signal. a change in a predetermined sense in the potential applied by the frequency detector I6 to the control electrode of the variable-resistance amplifier Il and causes a change in the opposite sense in the potential applied to the control :'f

electrode of the variable-resistance amplifier 21. Consequently, the values of resistance of the amplifiers I'I and 2l change in opposite senses with relation to the initial values thereof. The frequencies of the output signals of the oscillators I8 and 28 change accordingly, one decreasing and the other increasing. After amplication by amplifiers and 29, respectively, these signals are applied to the energizing circuits of the synchronous motors 26 and 3l) and cause the rotational speed of one of the motors to increase and the speed of the other motor to decrease. Thus the frequency converter I4, the intermediate-frequency amplifier I5 and the frequency detector I6 apply the derived control effect to the variable-frequency oscillators I8 and 28 to control the frequencies of the output signals thereof and to determine the rotational speeds of at least one of the pair of motors 26 and 30.

Accordingly, the motors 26 and 30 cause movement of the rotor of the condenser I2 in a predetermined direction through the action of the differential gear mechanism 32. The rotor of the condenser I2 is thereby so rotated that the oscillator I I is tuned to a lower frequency. When the rotor of the condenser I2 has been sufficiently displaced to tune the oscillator II to the desired lower frequency, the mode of operation is as previously described, namely the synchronous motors 26 and 30 then continuously rotate at the same speeds and the rotor of the condenser I2 remains motionless. Since the motors continuously rotate there is very little inertia in the servo system and accordingly, there is little time delay in the response of the system to changes in the control effect. Thus the system has little tendency to overrun and cause hunting A similar mode of operation occurs when a crystal having a resonant frequency higher than This control effect provides 6 that of the first crystal is selected for the local oscillator I0. The control effect then derived by the frequency converter I4, the intermediatefrequency amplifier I5 and the frequency detector I6 is applied to the variable-frequency oscillators I8 and 28 so to control the synchronous motors 26 and 30 as to provide a predetermined frequency difference between the first and second signals and thereby tune the oscillator II to a desired higher frequency. During this mode of operation the motors 26 and 30 continue to rotate in the same directions as previously, but the speed of one of the motors now decreases and the speed of the aforesaid other motor increases. Consequently the motors 26 and 30 cause movement of the rotor of the condenser I2 in a direction opposite to the above-mentioned predetermined direction and the oscillator II is thereby tuned to the desired higher frequency.

Further, it may be seen that if the frequency of the output signal of the oscillator Il varies from a predetermined or desired operating frequency due to variations in operating conditions, the servo system is effective similarly to retune that oscillator to substantially the predetermined operating frequency.

Under such operating conditions, means including the local oscillator Il), the frequency converter I4, the intermediate-frequency amplifier I5, and the frequency detector I6 derives a control effect related to the frequency variation of the output signal of the master oscillator II and applies that control effect to the oscillators I8 and 28 so to control the synchronous motors 26 and 3Q as to maintain the output signal of the master oscillator II at substantially the abovementioned predetermined frequency.

From the foregoing description of the invention, it will be apparent that a servo system embodying the invention has the advantage that two continuously rotatable synchronous motors respon-d with only slight time delay to a control effect derived from the output signal of the master oscillator automatically to tune that oscillator to a desired frequency.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modiiications may be made therein without departing from the invention, and it is, therefore, aimed to cover al1 such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

A servo system for distance measuring equipment comprising: a crystal-controlled oscillator for supplying a first signal having a substantially constant frequency; a tunable oscillator for supplying a second signal including a frequencyadjusting device; a pair of continuously rotatable synchronous motors; a differential gear mechanism for differentially connecting said pair of synchronous motors to said adjusting device to control the frequency of said second signal; a pair of variable-frequency oscillators including a pair of frequency-determining networks and coupled to the energizing circuits of said pair of synchronous motors for providing a pair of output signals to energize said pair of synchronous motors; a frequency converter coupled to said crystal-controlled oscillator and said tunable oscillator for heterodyning said first and second signals to provide a signal having a frequency equal to the frequency difference between Saidlfnst and second signals; @frequency detector GQunledmQ said; frequency com/,enter and said pair of.y vaarahle-freuency oscillators fer, deriving a central; effect: from seid dierence-frequeney signal: and a@ pai-.n of; varirkzle-:wesisnemee1 amplifiers Coupled between sal@ frequency detecter and;V said pair of fxequencwdetermining netwoxke. for utilizing said contlol. eiect t0,- Qauee said; pedirl of variable-frequency oscillators, so,- to comsrol the frequencies of said Qutpun signals lhereqi1 as.; to deter-minethev xscljseui@nait` speeds; 0i saidf 4syncm'ormusl mations: and; thereby provide an mede- Refer-enges in. the leof 'chiel miem. ImLTED STATES mfrxmrfvsl 

